Use default commit.h file if git index is not available

BUILD.gn

@@ -5,6 +5,15 @@
 # import the use_x11 variable
 import("//build/config/ui.gni")
 
+angle_git_is_present = exec_script("src/commit_id.py",
+                                   [
+                                     "check",
+                                     rebase_path(".", root_build_dir),
+                                   ],
+                                   "value")
+
+angle_use_commit_id = angle_git_is_present == 1
+
 gles_gypi = exec_script(
   "//build/gypi_to_gn.py",
   [ rebase_path("src/libGLESv2.gypi") ],
@@ -184,19 +193,32 @@ config("commit_id_config") {
   include_dirs = [ "$root_gen_dir/angle" ]
 }
 
-action("commit_id") {
-  script = "src/commit_id.py"
-
-  output_file = "$root_gen_dir/angle/id/commit.h"
-  outputs = [ output_file ]
+commit_id_output_file = "$root_gen_dir/angle/id/commit.h"
+if (angle_use_commit_id) {
+  action("commit_id") {
+    script = "src/commit_id.py"
+    outputs = [
+      commit_id_output_file,
+    ]
 
-  args = [
-    "gen",
-    rebase_path(".", root_build_dir),
-    rebase_path(output_file, root_build_dir),
-  ]
+    args = [
+      "gen",
+      rebase_path(".", root_build_dir),
+      rebase_path(commit_id_output_file, root_build_dir),
+    ]
 
-  public_configs = [ ":commit_id_config" ]
+    public_configs = [ ":commit_id_config" ]
+  }
+} else {
+  copy("commit_id") {
+    sources = [
+      "src/commit.h",
+    ]
+    outputs = [
+      commit_id_output_file,
+    ]
+    public_configs = [ ":commit_id_config" ]
+  }
 }
 
 config("libANGLE_config") {

CONTRIBUTORS

@@ -104,3 +104,4 @@ NVIDIA Corporation
 
 Opera Software ASA
  Daniel Bratell
+ Tomasz Moniuszko